1. Field of the Invention
The present invention relates to an exposure apparatus and a method of manufacturing a device.
2. Description of the Related Art
Semiconductor devices such as an IC and LSI, liquid crystal devices, image sensing devices such as a CCD, and devices such as a magnetic head are manufactured using photolithography. At this time, the pattern of a reticle (to be also referred to as an original or mask hereinafter) is projected onto a photosensitive substrate (to be also simply referred to as a substrate hereinafter) such as a semiconductor wafer at a predetermined magnification, thereby exposing the photosensitive substrate. Along with the recent increase in the packing density of integrated circuits (devices) such as an IC and LSI, an exposure apparatus which can precisely form a micropattern with a minimum line width as small as several tens of nanometers on the wafer has come to be used.
In general, an appropriate exposure amount is determined for the photosensitive agent applied on the wafer. To attain this amount, techniques of controlling the exposure amount to expose the wafer with a predetermined exposure amount have conventionally been proposed. To control the exposure amount, an optical member such as a half mirror is normally inserted in the optical path of the exposure light. A detection unit for monitoring the exposure amount, such as an optical sensor, receives one of light reflected by the optical member or that transmitted through the optical member, thereby detecting the exposure amount. In accordance with the signal output from the detection unit, the exposure amount is controlled by, for example, the opening/closing of a shutter inserted in the optical path of the exposure light, the emission/stop of the laser, or the control of energy set for the laser. Japanese Patent No. 2785157 discloses details of such a technique.
Along with the recent advance in micropatterning of semiconductor devices, a polarization illumination system has come to be used. In this system, the wafer is exposed with P- or S-polarized exposure light or exposure light having P- and S-polarized light components at an arbitrarily set ratio. When the wafer is exposed using the polarization illumination system, the exposure amount is controlled using the detection unit, as mentioned above, as well.
To control the exposure amount so as to obtain an optimum exposure amount on the wafer surface, a method is available which inserts an optical member such as a half mirror in the optical path of the exposure light, and detects the exposure light divided via the optical member by a detection unit for monitoring the exposure amount, such as an optical sensor, thereby controlling the exposure amount. The optical member generally has different reflectances and transmittances with respect to the P- and S-polarized light components of the exposure light. Moreover, depending on the polarization characteristic of an optical member inserted in the optical path up to the wafer surface, the ratio between the P- and S-polarized light components of the exposure light divided by the optical member differs between the light-receiving surface of a detection unit such as an optical sensor which measures the exposure amount, and the wafer surface irradiated with the exposure light. Especially when an optical member inserted in the optical path up to the light-receiving surface of the detection unit such as an optical sensor is different in property from that inserted in the optical path up to the wafer surface, the ratio between the polarized light components may largely differ between the light-receiving surface and the wafer surface.
In this manner, when the polarization state of a laser serving as the light source changes over time or when the polarization characteristics of constituent optical members change, the ratio between the amount of light which becomes incident on the detection unit such as an optical sensor and the exposure amount on the wafer surface fluctuates. Therefore, the value detected by the detection unit is insufficient to precisely monitor the amount of exposure light which becomes incident on the wafer surface.
In a polarization illumination system of exposing the wafer with P- or S-polarized exposure light or exposure light having P- and S-polarized light components at an arbitrarily set ratio, the ratio between the amount of light which becomes incident on the detection unit such as an optical sensor and the exposure amount on the wafer surface fluctuates more largely than that in the random polarization illumination system. This makes it more difficult to precisely monitor the exposure amount.
Conventionally, the exposure light divided by a half mirror serving as a first light beam dividing unit is further divided using a second light beam dividing unit for polarization division, such as a polarizing beam splitter or Rochon prism, and the exposure amount is measured and controlled by taking account of the polarized light components of the exposure light. Japanese Patent Laid-Open Nos. 2002-198281 and 2004-37137 discloses details of such techniques.
A polarizing beam splitter used herein must be fabricated by taking account of conditions such as the transmittances and reflectances of the P- and S-polarized light components, assuming various polarization states and various illumination conditions such as annular illumination, dipole illumination, and quadrupole illumination. For this reason, the polarizing beam splitter used herein is more expensive than a general-purpose polarizing beam splitter. A Rochon prism can divide the P- and S-polarized light components at a good transmittance ratio, thereby controlling the exposure amount. However, to obtain a good transmittance ratio between the P- and S-polarized light components the Rochon prism requires an accommodation space larger than that for the polarizing beam splitter. Furthermore, there is a limitation of the incident angle of a light beam with respect to the Rochon prism, so the degree of freedom of its arrangement is low.